Question (i 'm not signed up on g+ yet): People seem to be calling circles "groups" while they seem more like the "friend lists" of facebook. Does G+ have discussion groups that anyone can join (and start a twitter revolution)?
Hm, well, your stream gives you a way to select which circles you are viewing, almost like tabs. I'm not sure I'd call it a competition with Twitter, though, more just a filter on top of streams in this case. Though, we have yet to see what can really be accomplished with streams.
I think circles/"groups" are no different to categories in your contact list